hostapd: build the variants in parallel

The variants only write to their own build directories. The packages
without a variant are only built by the first variant.

With all packages selected, a clean build of the 31 variants takes 46 s
instead of 114 s, and the packages do not change. Together with the
commit that lists every build variant only once, a build that has
nothing to do takes 17 s instead of 33 s, as the variants were the
longest chain of it.

Signed-off-by: John Crispin <john@phrozen.org>

package/network/services/hostapd/Makefile

@@ -18,6 +18,7 @@ PKG_LICENSE:=BSD-3-Clause
 PKG_CPE_ID:=cpe:/a:w1.fi:hostapd
 
 PKG_BUILD_PARALLEL:=1
+PKG_PARALLEL_VARIANTS:=1
 PKG_ASLR_PIE_REGULAR:=1
 
 PKG_CONFIG_DEPENDS:= \